Malfunction detecting circuit and malfunction detecting method for detecting malfunction of current-sensing resistor, and power converting system applying the malfunction detecting circuit
A malfunction detecting circuit for detecting malfunction of a current sensing resistor includes a reference-voltage generating circuit and a comparing circuit, wherein the reference-voltage generating circuit is utilized to generate a reference voltage signal varying with the on time period of a power switch of a power converting system. The comparing circuit compares a sensing voltage signal corresponding to a current flowing through the current sensing resistor with the reference voltage signal to generate a comparing result indicating whether the malfunction occurs.
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1. Field of the Invention
The present invention relates to a malfunction detecting circuit implemented in a power converting system and a detecting method thereof, and more particularly, to a malfunction detecting circuit for detecting malfunction of a current sensing resistor in a power converting system, and a detecting method thereof.
2. Description of the Prior Art
Sometimes malfunction, such as mechanical breakdown, improper operation, or any other accident, occurs and renders the resistance of the current sensing resistor Rcs almost zero, accidentally short the source pin of power switch Q1 to ground. Thus, the current sensing pin CS can no longer detect the over current situation of the primary winding current IP, and the control chip 110 may conduct the power switch Q1 at the maximum duty cycle, resulting in an unexpectedly-increasing output voltage VOUT, which may adversely affect the operation of the fly-back power converter 100. One solution to the detection of the malfunction is to detect the Vcc voltage provided by the auxiliary winding LAUX of the transformer T, and to determine if the output voltage VOUT is too high according to the detecting result. When the control chip 110 detects that the Vcc voltage is higher than a predefined over voltage protection threshold, it determines the output voltage VOUT is too high, and an over voltage protection mechanism should be activated.
In practice, the predefined over voltage protection threshold is much higher than the voltage of Vcc under normal operation, however. It will be complex and difficult to design the coil ratio of the primary winding LP to the auxiliary winding LAUX while giving consideration to both normal operation and over voltage protection. Moreover, when the output voltage VOUT initially increases from zero voltage level, the Vcc voltage has already been away from zero voltage level since it obtains part of its power directly from the input AC voltage VAC through a resistor R1. Hence, when the primary winding LP is suddenly stopped conducting current, the diode DSN at the secondary side becomes conductive earlier than the diode DA at the auxiliary side, causing that the secondary winding LS obtains power stored in the primary winding LP earlier than the auxiliary winding LAUX does. As a result, it may be deemed late to enable the over voltage protection only when Vcc voltage is detected to be higher than the predefined over voltage protection threshold, because the output voltage VOUT may be too high and cause damage to the circuits coupled to the output end of the fly-back power converter 100.
SUMMARY OF THE INVENTIONAccording to one exemplary embodiment of the present invention, a malfunction detecting circuit utilized to detect malfunction occurring to a current sensing resistor is disclosed. The malfunction detecting circuit comprises a reference-voltage generating circuit and a comparing circuit. The reference-voltage generating circuit generates a reference voltage signal varying with an on time period of a power switch, and the comparing circuit compares a sensing voltage signal with the reference voltage signal to generate a comparing result which indicates whether the malfunction occurs. The sensing voltage signal corresponds to a current flowing through the current sensing resistor.
According to another exemplary embodiment of the present invention, a power converting system is disclosed. The power converting system comprises a transformer having a primary winding, and a malfunction detecting circuit for detecting malfunction occurring to a current sensing resistor. The primary winding of the transformer is coupled to a power switch. The current sensing resistor is for sensing the current flowing through the primary winding. The malfunction detecting circuit comprises a reference-voltage generating circuit and a comparing circuit. The reference-voltage generating circuit generates a reference voltage signal varying with an on time period of the power switch. The comparing circuit compares a sensing voltage signal corresponding to a current flowing through the current sensing resistor with the reference voltage signal, to generate a comparing result indicating whether the malfunction occurs.
According to another exemplary embodiment of the present invention, a malfunction detecting method for detecting malfunction occurring to a current sensing resistor is disclosed. The method comprises generating a reference voltage signal varying with an on time period of a power switch, comparing a sensing voltage signal with the reference voltage signal in order to generate a comparing result, and determining whether the malfunction occurs according to the comparing result, wherein the sensing voltage signal corresponds to a current flowing through the current sensing resistor.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
Please refer to
The sensing voltage signal VCS corresponds to the current flowing through the current sensing resistor; that is, it corresponds to the voltage signal detected by the current sensing (CS) pin. Because the voltage signal VCS detected by the CS pin is usually small in amplitude and not easy to be compared to the reference voltage signal, the malfunction detecting circuit 200 can further comprise an amplifier 230 coupled to the CS pin, as shown in
The noise level of the voltage signal VCS is decreased after the voltage signal VCS itself is processed by both the amplifier 230 and the sample-and-hold circuit 240. Therefore, the probability that the comparing circuit 220 mistakenly determines the comparing result between the voltage levels of the sensing voltage signal VS and the reference voltage signal VR can be decreased. The amplifier 230 and the sample-and-hold circuit 240, however, are not necessary elements of the present invention; other adjusting circuits that are able to raise the SNR or detecting accuracy can replace the amplifier 230 and/or the sample-and-hold circuit 240. Moreover, if the signal VCS detected by the current sensing resistor has good signal quality, the signal VCS can be directly utilized as the sensing voltage signal VS mentioned above, as long as the reference voltage signal VR has been properly designed.
Referring to
Vr/(R×C)<(Vin×RS×A)/LP,
wherein A represents the amplification factor of the amplifier 230 (in
During every on time period, ton, the comparing circuit 220 compares the sensing voltage signal VS with the reference voltage signal VR to determine whether the current sensing resistor malfunctions or not. In order to block the spike generated at the primary side of the transformer due to the suddenly turn-on of the power switch, the comparing circuit 220 can be further coupled to a leading edge blanking circuit 250 at the output end receiving VCS. The leading edge blanking circuit 250 can blank the comparing result of the comparing circuit 220 during a short blanking time period starting at the conducting of the power switch, therefore the malfunction detecting circuit 220 only replies to the comparing result normally after the blanking time period passes. Since the transformer will not enter saturation as soon as the power switch becomes conductive during the blanking time period, a malfunction current sensing resistor will not result in over current to damage the power converting system during the blanking time period. After the blanking time period passes, the power converting system is protected by the enabling of the malfunction protection mechanism.
Therefore, in the situation that the current sensing resistor is functionally normal, the sensing voltage signal VS will be more than or equal to the reference voltage signal VR generated by integrating the fixed voltage Vr. However, when the current sensing resistor is malfunctioning, having resistance so small that it almost shorts one terminal of the power switch such as Q1 of
In one embodiment, the malfunction detecting circuit 200 can be integrated in the control chip 110 of the power converting system shown in
Moreover, the trigger condition of the malfunction protection mechanism is adaptive to the system status because the reference voltage signal VR varies with the on time period ton of the power switch. The malfunction detecting circuit 200 is therefore more flexible and simpler than the malfunction detecting circuit that uses fixed over voltage protection threshold. For example, when the power converting system has a light load, the Vcs signal is usually small; however, the time period for the reference-voltage generating circuit 210 to integrate the fixed voltage Vr is also short since the on time period ton of the power switch is short under the condition of light load, and the reference voltage signal VR generated by the reference-voltage generating circuit 210 is therefore still less than the sensing voltage signal VS. As a result, the malfunction detecting circuit 200 can always correctly detect whether the malfunction of the current sensing resistor occurs.
To summarize, by utilizing a reference voltage signal varying with the conducting time period of a transformer as the determining basis of enabling an malfunction protection mechanism, embodiments of the present invention can detect the malfunction of a current sensing resistor in time so that a protection mode is enabled before the saturation of the transformer thereby preventing the elements such as the transistor or chip from damage. Embodiments of the present invention also have the advantages of simple circuit structure and easy implementation.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.
Claims
1. A malfunction detecting circuit for detecting malfunction occurring to a current sensing resistor, comprising:
- a reference-voltage generating circuit, for generating a reference voltage signal varying with an on time period of a power switch, wherein when the power switch is turned off, the reference voltage signal is zero; and
- a comparing circuit, for comparing a sensing voltage signal with the reference voltage signal to generate a comparing result indicating whether the malfunction occurs, wherein the sensing voltage signal corresponds to a current flowing through the current sensing resistor.
2. The malfunction detecting circuit of claim 1, wherein the comparing result indicates the malfunction occurs when the sensing voltage signal has signal amplitude smaller than the reference voltage signal.
3. The malfunction detecting circuit of claim 1, wherein the reference-voltage generating circuit comprises an integrator, for integrating a fixed voltage to generate the reference voltage signal when the power switch is on.
4. The malfunction detecting circuit of claim 3, wherein the integrator is controlled by a pulse width modulation (PWM) signal utilized to control the on time period of the power switch in a power converting system.
5. The malfunction detecting circuit of claim 3, wherein a start time point of the integrator is synchronized to a time point at which the power switch begins to be on.
6. The malfunction detecting circuit of claim 1, wherein the comparing circuit compares the sensing voltage signal to the reference voltage signal when the power switch is on.
7. The malfunction detecting circuit of claim 1, further comprising an amplifier, for amplifying a voltage detected by the current sensing resistor to generate the sensing voltage signal.
8. The malfunction detecting circuit of claim 1, further comprising a sample-and-hold circuit, for sampling and holding a peak value of a voltage detected by the current sensing resistor to generate the sensing voltage signal.
9. The malfunction detecting circuit of claim 8, wherein the sample-and-hold circuit is controlled by a pulse width modulation (PWM) signal utilized to control the on time period of the power switch in a power converting system.
10. The malfunction detecting circuit of claim 1, wherein the comparing result causes the power switch to be kept off when the malfunction occurs.
11. The malfunction detecting circuit of claim 1, further comprising a blanking circuit, for blanking the malfunction detecting circuit from enabling a malfunction protection mechanism during a blanking time period starting when the power switch is on.
12. A power converting system, comprising:
- a transformer comprising a primary winding; and
- a malfunction detecting circuit of claim 1, wherein the power switch is coupled to the primary winding of the transformer, and the current sensing resistor senses a current flowing through the primary winding.
13. The power converting system of claim 12, wherein the comparing result causes the power switch to be kept off when the malfunction occurs.
14. A method for detecting malfunction occurring to a current sensing resistor, comprising:
- generating a reference voltage signal varying with an on time period of a power switch, wherein when the power switch is turned off, the reference voltage signal is zero; and
- comparing a sensing voltage signal with the reference voltage signal to generate a comparing result, and determining whether the malfunction occurs according to the comparing result, wherein the sensing voltage signal corresponds to a current flowing through the current sensing resistor.
15. The method of claim 14, wherein the comparing result indicates the malfunction occurs when the sensing voltage signal has signal amplitude smaller than the reference voltage signal.
16. The method of claim 14, wherein the step of generating the reference voltage signal comprises integrating a fixed voltage to generate the reference voltage signal when the power switch is on.
17. The method of claim 16, wherein the step of integrating the fixed voltage is controlled by a pulse width modulation (PWM) signal utilized to control the on time period of the power switch in a power converting system.
18. The method of claim 14, wherein the step of generating the comparing result comprises comparing the sensing voltage signal with the reference voltage signal when the power switch is on.
19. The method of claim 14, wherein the comparing result causes the power switch to be kept off when the malfunction occurs.
20. The method of claim 14, wherein the step of comparing the sensing voltage signal to the reference voltage signal is performed after a blanking time period starting when the power switch is on.
20070127170 | June 7, 2007 | Park |
Type: Grant
Filed: Oct 28, 2008
Date of Patent: Mar 8, 2011
Patent Publication Number: 20090212787
Assignee: Leadtrend Technology Corp. (Science-Based Industrial Park, Hsin-Chu)
Inventor: Chun-Liang Lin (Hsin-Chu)
Primary Examiner: Vincent Q Nguyen
Attorney: Winston Hsu
Application Number: 12/260,076
International Classification: G01R 31/02 (20060101);